POE::Wheel::FollowTail(3) User Contributed Perl Documentation POE::Wheel::FollowTail(3)

POE::Wheel::FollowTail - follow the tail of an ever-growing file

use POE qw(Wheel::FollowTail);
  inline_states => {
    _start => sub {
      $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
        Filename => "/var/log/system.log",
        InputEvent => "got_log_line",
        ResetEvent => "got_log_rollover",
    got_log_line => sub {
      print "Log: $_[ARG0]\n";
    got_log_rollover => sub {
      print "Log rolled over.\n";

POE::Wheel::FollowTail objects watch for new data at the end of a file and generate new events when things happen to the file. Its "Filter" parameter defines how to parse data from the file. Each new item is sent to the creator's session as an "InputEvent" event. Log rotation will trigger a "ResetEvent".

POE::Wheel::FollowTail only reads from a file, so it doesn't implement a put() method.

new() returns a new POE::Wheel::FollowTail object. As long as this object exists, it will generate events when the corresponding file's status changes.

new() accepts a small set of named parameters:


The optional "Driver" parameter specifies which driver to use when reading from the tailed file. If omitted, POE::Wheel::FollowTail will use POE::Driver::SysRW. This is almost always the right thing to do.


"Filter" is an optional constructor parameter that specifies how to parse data from the followed file. By default, POE::Wheel::FollowTail will use POE::Filter::Line to parse files as plain, newline-separated text.

$_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
  Filename => "/var/log/snort/alert",
  Filter => POE::Filter::Snort->new(),
  InputEvent => "got_snort_alert",


POE::Wheel::FollowTail needs to periodically check for new data on the followed file. "PollInterval" specifies the number of seconds to wait between checks. Applications that need to poll once per second may omit "PollInterval", as it defaults to 1.

Longer poll intervals may be used to reduce the polling overhead for infrequently updated files.

$_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
  PollInterval => 10,


If specified, "Seek" instructs POE::Wheel::FollowTail to seek to a specific spot in the tailed file before beginning to read from it. A positive "Seek" value is interpreted as the number of octets to seek from the start of the file. Negative "Seek" will, like negative array indices, seek backwards from the end of the file. Zero "Seek" starts reading from the beginning of the file.

Be careful when using "Seek", as it's quite easy to seek into the middle of a record. When in doubt, and when beginning at the end of the file, omit "Seek" entirely. POE::Wheel::FollowTail will seek 4 kilobytes back from the end of the file, then parse and discard all records unto EOF. As long as the file's records are smaller than 4 kilobytes, this will guarantee that the first record returned will be complete.

"Seek" may also be used with the wheel's tell() method to restore the file position after a program restart. Save the tell() value prior to exiting, and load and "Seek" back to it on subsequent start-up.


"SeekBack" behaves like the inverse of "Seek". A positive value acts like a negative "Seek". A negative value acts like a positive "Seek". A zero "SeekBack" instructs POE::Wheel::FollowTail to begin at the very end of the file.

"Seek" and "SeekBack" are mutually exclusive.

See "Seek" for caveats, techniques, and an explanation of the magic that happens when neither "Seek" nor "SeekBack" is specified.


POE::Wheel::FollowTail may follow a previously opened file "Handle". Unfortunately it cannot follow log resets this way, as it won't be able to reopen the file once it has been reset. Applications that must follow resets should use "Filename" instead.

"Handle" is still useful for files that will never be reset, or for devices that require setup outside of POE::Wheel::FollowTail's purview.

"Handle" and "Filename" are mutually exclusive. One of them is required, however.


Specify the "Filename" to watch. POE::Wheel::FollowTail will wait for the file to appear if it doesn't exist. The wheel will also reopen the file if it disappears, such as when it has been reset or rolled over. In the case of a reset, POE::Wheel::FollowTail will also emit a "ResetEvent", if one has been requested.

"Handle" and "Filename" are mutually exclusive. One of them is required, however.

See the "SYNOPSIS" for an example.


"IdleEvent" is an optional event. If specified, it will fire whenever POE::Wheel::FollowTail checks for activity but sees nothing. It was added in POE 1.362 as a way to advance certain test programs without needing to wait conservatively large amounts of time.

"IdleEvent" is described in "PUBLIC EVENTS".


The "InputEvent" parameter is required, and it specifies the event to emit when new data arrives in the watched file. "InputEvent" is described in detail in "PUBLIC EVENTS".


"ResetEvent" is an optional. It specifies the name of the event that indicates file rollover or reset. Please see "PUBLIC EVENTS" for more details.


POE::Wheel::FollowTail may emit optional "ErrorEvent"s whenever it runs into trouble. The data that comes with this event is explained in "PUBLIC EVENTS".

event() allows a session to change the events emitted by a wheel without destroying and re-creating the object. It accepts one or more of the events listed in "PUBLIC EVENTS". Undefined event names disable those events.

Stop handling log resets:

sub some_event_handler {
  $_[HEAP]{tailor}->event( ResetEvent => undef );

The events are described in more detail in "PUBLIC EVENTS".

The ID() method returns the wheel's unique ID. It's useful for storing the wheel in a hash. All POE::Wheel events should be accompanied by a wheel ID, which allows the wheel to be referenced in their event handlers.

sub setup_tailor {
  my $wheel = POE::Wheel::FollowTail->new(... incomplete ...);
  $_[HEAP]{tailors}{$wheel->ID} = $wheel;

See the example in "ErrorEvent" for a handler that will find this wheel again.

tell() returns the current position for the file being watched by POE::Wheel::FollowTail. It may be useful for saving the position program termination. new()'s "Seek" parameter may be used to resume watching the file where tell() left off.

sub handle_shutdown {
  # Not robust.  Do better in production.
  open my $save, ">", "position.save" or die $!;
  print $save $_[HEAP]{tailor}->tell(), "\n";
  close $save;
sub handle_startup {
  open my $save, "<", "position.save" or die $!;
  chomp(my $seek = <$save>);
  $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
    Seek => $seek,

POE::Wheel::FollowTail emits a small number of events.

"IdleEvent" specifies the name of an event to be fired when POE::Wheel::FollowTail doesn't detect activity on the watched file.

$_[ARG0] contains the ID of the POE::Wheel::FollowTail object that fired the event.

"InputEvent" sets the name of the event to emit when new data arrives into the tailed file. The event will be accompanied by two parameters:

$_[ARG0] contains the data that was read from the file, after being parsed by the current "Filter".

$_[ARG1] contains the wheel's ID, which may be used as a key into a data structure tracking multiple wheels. No assumption should be made about the nature or format of this ID, as it may change at any time. Therefore, track your wheels in a hash.

See the "SYNOPSIS" for an example.

"ResetEvent" names the event to be emitted whenever the wheel detects that the followed file has been reset. It's only available when watching files by name, as POE::Wheel::FollowTail must reopen the file after it has been reset.

"ResetEvent" comes with only one parameter, $_[ARG0], which contains the wheel's ID. See "InputEvent" for some notes about what may be done with wheel IDs.

See the "SYNOPSIS" for an example.

"ErrorEvent" names the event emitted when POE::Wheel::FollowTail encounters a problem. Every "ErrorEvent" comes with four parameters that describe the error and its situation:

$_[ARG0] describes the operation that failed. This is usually "read", since POE::Wheel::FollowTail spends most of its time reading from a file.

$_[ARG1] and $_[ARG2] contain the numeric and stringified values of $!, respectively. They will never contain EAGAIN (or its local equivalent) since POE::Wheel::FollowTail handles that error itself.

$_[ARG3] contains the wheel's ID, which has been discussed in "InputEvent".

This error handler logs a message to STDERR and then shuts down the wheel. It assumes that the session is watching multiple files.

sub handle_tail_error {
  my ($operation, $errnum, $errstr, $wheel_id) = @_[ARG0..ARG3];
  warn "Wheel $wheel_id: $operation error $errnum: $errstr\n";
  delete $_[HEAP]{tailors}{$wheel_id};

POE::Wheel describes the basic operations of all wheels in more depth. You need to know this.

The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.

This wheel can't tail pipes and consoles on some operating systems.

POE::Wheel::FollowTail generally reads ahead of the data it returns, so the tell() position may be later in the file than the data an application has already received.

Please see POE for more information about authors and contributors.

2024-07-13 perl v5.38.2